Recently, desalting seawater through a composite semipermeable membrane has been tried, and now, it has been put into practical use in water plants all over the world. A composite semipermeable membrane generally comprises a separating functional layer coated on a porous substrate film. When the separating functional layer is formed by using crosslinked aromatic polyamide, there are such advantages that the layer is stiff and rigid since it contains a benzene ring and that the layer can be readily formed by interfacial polycondensation of aromatic polyfunctional amine and aromatic polyfunctional acid halide, and other advantages such as a high salt removal ratio and a high permeation flow rate are known (JP-A-1-180208 and JP-A-2-115027).
However, the water quality standard in the art is controlled more and more severely these days. In particular, it is difficult to reduce the minor boron in seawater to a level acceptable for drinking water in ordinary treatment, and some composite semipermeable membranes have been proposed for solving this problem (JP-A-11-19493 and JP-A-2001-259388). However, since these membranes are expected to have a membrane permeation flow rate of 0.5 m3/m2/day or less and a boron removal ratio at most about 91 to 92% when seawater at 25° C. having a pH of 6.5, a boron concentration of 5 ppm and a TDS concentration of 3.5% by weight is permeated under an operation pressure of 5.5 MPa, development of composite semipermeable membranes which have higher solute-rejection performance has been desired.
As means for improving the solute-rejection performance of composite semipermeable membranes, a method in which a novel reactant is added to the reaction solution is exemplified. Since this method does not so much differ from conventional methods, it is useful as a simple improved method. For example, it is described that in the composite semipermeable membrane, crosslinked polyamide is constituted by a polyamine component having at least two amino groups in the molecule, and an acid component, as a novel reactant, which comprises a linear aliphatic poly-acid halide having at least two halogenocarbonyl groups in the molecule (Japanese Patent 3,031,763). Although it is described that the method provides a composite semipermeable membrane having a high salt removal ratio and a high permeation flow rate and a production process thereof, it is still impossible to further increase the boron rejection ratio according to the method.
On the other hand, it has also been attempted to conduct processing of bringing a reagent into contact with a semipermeable membrane to improve the selective separating performance and improve the retainability thereof. For example, a method of bringing a semipermeable membrane into contact with two or more kinds of aqueous solutions containing water soluble compounds reactive to each other successively can be mentioned (JP-A-59-115704). While the method is useful for the improvement of long time stability of the desalting ratio of the semipermeable membrane, the boron removal ratio has not yet been improved to reach as high as 95%. Further, a method of using an anionic surfactant for the post-treatment is also mentioned (JP-A-62-110706). While the method is useful for suppressing lowering of the permeation flow rate in the sterilization of the semipermeable membrane, the boron removal ratio has not yet been improved to reach as high as 95%.
Further, a method of coating the surface of a separating functional polyamide layer with a solution of a compound having at least one aldehyde group is also mentioned (JP-A-2004-243198). While the method is useful for attaining aging-stability and a high permeation flow rate, the boron removal ratio has not yet been improved to reach as high as 95%. Further, while a method of acting an amine reactive reagent or an oxidizing agent to a polyamide composite membrane can also be mentioned (U.S. Pat. No. 4,964,998), the boron removal ratio has not yet been improved to reach as high as 95%.